1. Field of the Invention
The present invention is directed to digital telecommunications and, more particularly, communications between a plurality of terminal devices, such as digital telephones, and a central office of the public switched telephone network.
2. Description of the Related Art
Digital transmission of both voice and data is used extensively in the public switched telephone networks (PSTNs) worldwide. In North America, most systems transmit at a basic rate of 1.544 million bits per second (Mbps) or a multiple thereof. Elsewhere in the world, 2.048 Mbps is widely used as the basic rate. These rates are common due to international standardization and the availability of electronics for switching, multiplexing, encoding and decoding at these rates.
The transmission media used for digital telecommunication include twisted-pair wires, microwaves and optical fibers. Two sets of twisted-pair wires are used for full-duplex transmission and full-duplex microwave transmission uses two frequency pairs. A single optical fiber may be used for full-duplex transmission at the basic rate and at much higher rates, e.g., 90 or 180 Mbps.
In addition to differences in the rate of transmission, several different transmission protocols are used throughout the world. At the present time in North America, T1 spans using the extended superframe (ESF) protocol connect many of the central office switches in the PSTN. The superframe (SF) protocol is currently more common between the PSTN and digital private branch exchanges (PBXs), but ESF is available for these links which are discussed below in more detail. The Consultative Committee for International Telephone and Telegraph (CCITT) has several digital protocols. The G.700 Series known as E1 or CEPT-1 multiplexes thirty 8-bit subscriber channels with an 8-bit synchronization and alarm channel and an 8-bit signalling channel for a total of thirty-two channels transmitted together at 2.048 Mbps. The Integrated Services Digital Network (ISDN) transmission protocol is used for transmission between ISDN network terminal interfaces (NTIs) over a PSTN and is transmitted inside a protocol like T1 or E1.
Although digital lines are commonly used between central office switches and digital telephones are available from many sources (without standardization), there is relatively little use of all digital transmission from telephone to telephone via the PSTN. The vast majority of residences and many businesses, particularly those smaller in size, use analog telephones which may be connected to a remote terminal that performs analog/digital conversion for telephones and communicates to a central office in the PSTN via a digital line, such as a T1 span in the U.S. Digital PBXs are commonly used to connect digital telephones to the PSTN. However, PBXs typically operate at a multiple of a 12 KHz voice data transmission rate and use proprietary signalling protocols. As a result, digital PBXs buffer signals between digital telephones and the PSTN to perform timing and protocol conversions.
Furthermore, most central office switches in North America provide a predetermined number of embedded derived carrier systems, e.g., sixty-four SLC.RTM.96s, but do not provide an inherent ability to connect to PBXs, in part because the requirements of PBXs can vary. In addition, due to the buffering and signalling conversion described above, conventional PBXs cannot easily be connected to a SLC.RTM.96 which includes derived data link (DDL) signalling information, i.e., maintenance, alarm, and in mode 2 concentration routing, on only one of the four T1 spans used to transmit voice signals. As a result, conventional PBXs are typically connected to central office switches in the PSTN using additional equipment in the central office to provide digital trunk connections, or using analog line connections and analog/digital converters for digital PBXs.
A station line controller as disclosed in U.S. Pat. No. 5,027,341, incorporated by reference herein, enables digital telephones to be connected via a multiplexed digital channel to a central office in the PSTN without any digital/analog conversion and without using a remote terminal. A standard subscriber carrier system like SLC.RTM.96 may be used to provide four T1 spans plus one spare T1 span. The T1 spans provided by an SLC.RTM.96 will be referred to as A, B, C and D links to distinguish from the twenty-four subscriber channels on each link, although Conventionally they are usually referred to as A-D channels. U.S. Pat. No. 5,027,341 discloses that four station line controllers, each connected to up to 24 telephones, can be linked together to transfer the switching information for the SLC.RTM.96 which is transmitted only over the A link and to permit transfers of calls between any of the up to 96 telephones connected to the station controllers.
However, there are many uses for the station line controller disclosed in U.S. Pat. No. 5,027,341 which are not suggested therein. No other known device is capable of connecting digital telephones to the line side of a central office in the PSTN. Digital PBXs like all PBXs are connected to the trunk side of a central office. As a result, no other known device is able to take advantage of existing signalling on digital subscriber channels.